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False diffusion in numerical simulation of combustion processes in tangential-fired furnace
Xuchang Xu,Zhigang Wang,Yuqun Zhuo,Changhao Zheng 대한기계학회 2007 JOURNAL OF MECHANICAL SCIENCE AND TECHNOLOGY Vol.21 No.11
Numerical simulation serves as one of the most important tools for analyzing coal combustion in Tangentially Fired Furnaces (TFF) with NUMERICAL FALSE DIFFUSION as one key problem that degrades the simulation accuracy, especially for complex flow patterns. False diffusion often completely compromises the accuracy, leading to erroneous predictions. This paper reviews various methods to reduce the numerical diffusion. In computational fluid dynamics (CFD), false diffusion originates from a truncation error of the Taylor series approximation of the derivative and multidimensional discretization effects. Higher-order upwind convective schemes were designed to reduce truncation errors, while grid line adjusting methods were developed to reduce crossflow diffusion. This paper compares numerical and experimental results for isothermal flows to evaluate these methods. Results with the standard upwind scheme in a rectangular Cartesian mesh are compared with results in body-fitted meshes for comprehensive combustion processes in a TFF. Analysis of the false diffusion effect in the x, y, z directions and the artificial viscosity distribution in a rectangular mesh shows where the false diffusion overtakes the real physical diffusion and where the mesh must be refined or grid line must be adjusted to improve TFF combustion simulations.
Catalytic reduction of nitric oxide by methane over CaO catalyst
Wei Xu,Huiling Tong,Changhe Chen,Xuchang Xu 한국화학공학회 2008 Korean Journal of Chemical Engineering Vol.25 No.1
The selective catalytic reduction of nitric oxide by methane was studied over CaO catalyst in a bubbling fluidized bed in the temperature range of 800-900 oC, in which NO cannot be reduced by CH4 without CaO catalyst. The nitric oxide conversion was found to depend on oxygen and CH4 feed concentration, and also on temperature. In addition, the presence of water vapors in the flue gas enhanced the NO reduction admirably well in the absence of O2. But water vapor has an inhibiting effect on the reaction while O2 is present in the flue gas. The addition of CO2 poisoned the CaO catalyst and exhibited a detrimental effect on NO conversion at the working temperature range, 800-900 oC. However, with a temperature rise to 900 oC the CO2 poisoning effect on NO reduction was weakened. The mechanism was studied and discussed according to the references in the paper.
Life cycle emissions of greenhouse gas for ammonia scrubbing technology
Shujuan Wang,Fang Liu,Changhe Chen,Xuchang Xu 한국화학공학회 2007 Korean Journal of Chemical Engineering Vol.24 No.3
is thought that the CO2 emissions from coal-fired power plants contribute greatly to the total anthropo-genic CO2 emissions. Ammonia solvent can be used to absorb the CO2, caled amonia scrubbing. However, as hasbeen pointed out, the production of ammonia would emit CO2; therefore, the efectiveness of amonia scrubbing isdoubted. The paper focuses on the problem. Two systems are defined in the paper. System I is CO2 absorption by am-2 emissions of the twosystems are calculated by means of life cycle asessment. The paper shows that the total CO2 emissions of ammoniascrubbing are less than that of the industrial production of fertilizer ammonium bicarbonate. It can be concluded thatammonia scrubbing is an effective way to reduce the anthropogenic CO2 emissions.
Tianjin Li,Yuqun Zhuo,Junyong Lei,Xuchang Xu 한국화학공학회 2007 Korean Journal of Chemical Engineering Vol.24 No.6
potential of the sorbent-catalysts prepared from three low cost materials, i.e., the lime, fly ash andsome industrial waste material containing iron oxide, have been investigated for simultaneous removal of SO2 and NOxfrom flue gas in the temperature range 700-850oC. NH3 was chosen as the reducing agent for NO reduction in thisstudy. Experimental results showed that SO2 and NO could be simultaneously removed efficiently in the absence ofO2 at the temperature window of 700-800oC. The effect of product layer generated from SO2 removal on NO removal2, which was attributed to the partial oxidationof NH3 to NO over the sorbent-catalysts in the presence of oxygen. Neither NO2 nor N2O by-product was detected bothin the absence and presence of O2. Three routes were suggested to overcome the negative effect of O2.
Effects of water vapor, CO_2 and SO_2 on the NO reduction by NH_3 over sulfated CaO
Xinfang Yang,Bo Zhao,Yuqun Zhuo,Changhe Chen,Xuchang Xu 한국화학공학회 2011 Korean Journal of Chemical Engineering Vol.28 No.8
Gas effects on NO reduction by NH_3 over sulfated CaO have been investigated in the presence of O_2 at 700-850℃ . CO_2 and SO_2 have reversible negative effects on the catalytic activity of sulfated CaO. Although H_2O alone has no obvious effect, it can depress the negative effects of CO_2 and SO_2. In the flue gas with CO_2, SO_2 and H_2O coexisting,the sulfated CaO still catalyzed the NO reduction by NH_3. The in situ DRTFTS of H2O adsorption over sulfated CaO indicated that H_2O generated Brønsted acid sites at high temperature, suggesting that CO_2 and SO_2 competed for only the molecularly adsorbed NH_3 over Lewis acid sites with NO, without influencing the ammonia ions adsorbed over Brønsted acid sites. Lewis acid sites shifting to Brønsted acid sites by H_2O adsorption at high temperature may explain the depression of the negative effect on NO reduction by CO_2 and SO_2.
Quadruped Robot Control through Model Predictive Control with PD Compensator
Xu Chang,Hongxu Ma,Honglei An 제어·로봇·시스템학회 2021 International Journal of Control, Automation, and Vol.19 No.11
This paper proposes a control method when legs mass of a quadruped robot cannot be ignored or the robot carries heavy goods of unknown weight. The core part of the method is to solve the ground reaction forces on the foot ends. Expected ground reaction force is composed of two parts. The first part is given by the uniform force MPC controller. It uses a simplified robot dynamics equation to solve the ground reaction force received at each foot end. By unifying the input force at each time step, the solution time is shortened. The second part is given by a PD compensator, which is composed of force compensator and torque compensator. In order to verify the proposed method, a 12-degree-of-freedom quadruped robot model with a leg mass to body mass ratio of approximately 1:2 is established in the Gazebo simulation environment. The simulation results demonstrate that the method can realize the quadruped robot to traverse uneven terrain when carrying heavy objects and achieve various gait movements. In addition, the control method is robust and can be easily extended to legged robots.
Modeling of fiber pullout behaviors of stiff fiber reinforced cementitious composites
Xu Chang,Ya-Juan Chen,Hai-Xiao Lin,Yong-bin Zhang 사단법인 한국계산역학회 2012 Computers and Concrete, An International Journal Vol.9 No.3
This paper presents numerical studies of stiff fiber pullout behaviors of fiber reinforced cementitious composites based on a progressive damage model. The ongoing debonding process is simulated. Interfacial stress distribution for different load levels is analyzed. A parametric study, including bond strength and the homogeneity index on the pullout behaviors is carried out. The numerical results indicate that the bond stress decreases gradually from loaded end to embedded end along fiber-cement interface. The debonding initially starts from loaded end and propagates to embedded end as load increasing. The embedded length and bond strength affect the load-loaded end displacement curves significantly. The numerical results have a general agreement with the experimental investigation.